It is well known for lubricating oils to contain a number of surface active additives (including antiwear agents, dispersants, or detergents) used to protect internal combustion engines from corrosion, wear, soot deposits and acid build up. Often, such surface active additives can have harmful effects on engine component wear (in both iron and aluminium based components), bearing corrosion or fuel economy. A common antiwear additive for engine lubricating oils is zinc dialkyldithiophosphate (ZDDP). It is believed that ZDDP antiwear additives protect the engine by forming a protective film on metal surfaces. ZDDP may also have a detrimental impact on fuel economy and efficiency and copper corrosion. Consequently, engine lubricants may also contain a friction modifier to obviate the detrimental impact of ZDDP on fuel economy and corrosion inhibitors to obviate the detrimental impact of ZDDP on copper corrosion. Other additives may also increase lead corrosion.
Further, engine lubricants containing phosphorus compounds and sulphur have been shown to contribute, in part, to particulate emissions and emissions of other pollutants. In addition, sulphur and phosphorus tend to poison the catalysts used in catalytic converters, resulting in a reduction in performance of said catalysts.
With increasing control of both the formation of sulphated ash and release of emissions (typically to reduce NOx formation, SOx formation) there is a desire towards reduced amounts of sulphur, phosphorus and sulphated ash in engine oils. Consequently, the amounts of phosphorus-containing antiwear agents such as ZDDP, overbased detergents such as calcium or magnesium sulphonates and phenates have been reduced. As a consequence, ashless additives such as esters of polyhydric alcohols or hydroxyl containing acids including glycerol monooleate and alkoxylated amines have been contemplated to provide friction performance. However there have been observations that ashless friction modifiers may in some instances increase corrosion of metal, namely, copper or lead. Copper and lead corrosion may be from bearings and other metal engine components derived from alloys using copper or lead. Consequently, there is a need to reduce the amount of corrosion caused by ashless additives.
Canadian Patent CA 1 183 125 discloses lubricants for gasoline engines containing alkyl-ester tartrates, where the sum of carbon atoms on the alkyl groups is at least 8. The tartrates are disclosed as antiwear agents. Other references disclosing tartrates and/or tartrimides include International Publication WO 2006/044411, and US Patent Applications 2010/0190669, 2010/0197536, and 2010/0093573 for internal combustion engines requiring reduced amounts of sulphur, sulphated ash, and phosphorus. The lubricant composition has anti-wear or anti-fatigue properties. The lubricating compositions are suitable for road vehicles.
U.S. Pat. No. 4,237,022 discloses tartrimides useful as additives in lubricants and fuels for effective reduction in squeal and friction as well as improvement in fuel economy.
U.S. Pat. No. 5,338,470 and International Publication WO 2005/087904 disclose lubricants containing at least one hydroxycarboxylic acid ester or hydroxy polycarboxylic acid (in particular citrates or ethyl glycolate). The lubricant composition has anti-wear or anti-fatigue properties.
International Application WO2008/070307 discloses engine lubricants containing antiwear agents based on malonate esters.
Citrates are disclosed in US Patent Application 20050198894.
U.S. Pat. No. 4,436,640 discloses a lubricant antiwear agent prepared by a two step reaction involving (i) reacting glycolic acid with an alcohol containing 1 to 6 carbon atoms, and (ii) reacting the product of (i) with phosphorus pentasulphide. The antiwear agent is reported to be useful for a cam-follower set.
Lubricants additives derived from thioglycolic acid derivatives have been contemplated as additives. Additives from thioglycolic acid derivatives are summarised in a variety of U.S. Patents, Japanese patent application and an East German Patent. The U.S. Patents include U.S. Pat. Nos. 4,157,970, 4,863,622, 5,132,034, 5,215,549, and 6,127,327. The Japanese Patent Applications include 2005139238 A, Japanese Patent Applications 10183161A, 10130679A, and 05117680A. The East German Patent is DD 299533 A5.
For driveline power transmitting devices such as gears or transmissions, especially axle fluids and manual transmission fluids (MTFs), there are highly challenging technological problems and solutions for satisfying the multiple and often conflicting lubricating requirements, whilst providing durability and cleanliness. One of the important parameters influencing durability is the effectiveness of phosphorus antiwear or extreme pressure additives at providing devices with appropriate protection under various conditions of load and speed. However, many of the phosphorus antiwear or extreme pressure additives contain sulphur. Due to increasing environmental concerns, the presence of sulphur in antiwear or extreme pressure additives is becoming less desirable. In addition, many of the sulphur-containing antiwear or extreme pressure additives evolve sulphur due to numerous volatile sulphur species being present, resulting in lubricating compositions containing antiwear or extreme pressure additives having an odour and possibly also being detrimental to health and the environment.
A lubricating composition having the correct balance of phosphorus antiwear or extreme pressure additives provides driveline power transmitting devices with prolonged life and efficiency with controlled deposit formation and oxidation stability. However, many of the antiwear or extreme pressure additives employed have limited oxidative stability, form deposits or increase corrosion. In addition, many phosphorus antiwear or extreme pressure additives typically also contain sulphur, which results in a lubricating composition containing the phosphorus antiwear or extreme pressure additives which may be odorous.